Airway inflammation characterizes a number of severe lung diseases including asthma and chronic obstructive pulmonary disease (COPD). Events leading to airway obstruction include edema of airway walls, infiltration of inflammatory cells into the lung, production of various inflammatory mediators and increased mucous production. The airways of asthmatic patients are infiltrated by inflammatory leukocytes, of which eosinophils are the most prominent component. The magnitude of asthmatic reactions is correlated with the number of eosinophils present in the lungs.
The accumulation of eosinophils is found dramatically in the lungs of asthmatic patients although there are very few in the lungs of a normal individual. They are capable of lysing and activating cells and destroying tissues. When activated, they synthesize and release inflammatory cytokines such as IL-1, IL-3, and TNF-α and inflammatory mediators such as PAF, LTD4 and related oxygen species that can produce edema and broncho-constriction. Tumor necrosis factor (TNF-α) is also known to be involved in the pathogenesis of a number of autoimmune and inflammatory diseases. Consequently, manipulation of the cytokine signaling or biosynthetic pathways associated with these proteins may provide therapeutic benefit in those disease states. It has been well demonstrated that TNF-α production in pro-inflammatory cells becomes attenuated by an elevation of intracellular cyclic adenosine 3′,5′-monophosphate (cAMP). This second messenger is regulated by the phosphodiesterase (PDE) family of enzymes. The phosphodiesterase enzymes play an integral role in cell signaling mechanisms by hydrolyzing cAMP and cGP to their inactive 5′ forms. Inhibition of PDE enzymes thus results in an elevation of cAMP and/or cGP levels and alters intracellular responses to extra cellular signals by affecting the processes mediated by cyclic nucleotides. Since eosinophilis are believed to be a critical proinflammatory target for asthma, identification of the expression of the PDE 4 gene family in eosinophils led to PDE 4 as a potential therapeutic target for asthma [Rogers, D. F., Giembycz, M. A., Trends Pharmacol. Sci., 19, 160-164 (1998); Barnes, P. J., Trends Pharmacol. Sci., 19, 415-423 (1998)].
The mammalian cyclic nucleotide phosphodiesterases (PDEs) are classified into ten families on the basis of their amino acid sequences and/or DNA sequence, substrate specificity and sensitivity to pharmacological agents [Soderling, S. H., Bayuga, S. J., and Beavo, J. A., Proc. Natl. Acad. Sci., USA, 96, 7071-7076 (1999); Fujishige, K, Kotera, J., Michibata, H., Yuasa, K., Takebayashi, Si, Okamura, K. and Omori, K., J. Biol. Chem., 274, 18438-18445 (1999)]. Many cell types express more than one PDE and distribution of isoenzymes between the cells varies markedly. Therefore development of highly isoenzyme selective PDE inhibitors provides a unique opportunity for selective manipulation of various pathophysiological processes.
Phosphodiesterase type 4 (PDE4) is an enzyme which regulates activities in cells which lead to inflammation in the lungs. PDE4, a cAMP-specific and Ca+2-independent enzyme, is a key isozyme in the hydrolysis of cAMP in mast cells, basophils, eosinophils, monocytes and lymphocytes. The association between cAMP elevation in inflammatory cells with airway smooth muscle relaxation and inhibition of mediator release has led to widespread interest in the design of PDE4 inhibitors [Trophy, T. J., Am. J. Respir. Crit. Care Med., 157, 351-370 (1998)]. Excessive or unregulated TNF-α production has been implicated in mediating or exacerbating a number of undesirable physiological conditions such as osteoarthritis and other arthritic conditions, septic shock, endotoxic shock, respiratory distress syndrome and bone resorption diseases. Since TNF-α also participates in the onset and progress of autoimmune diseases, PDE4 inhibitors may find utility as therapeutic agents for rheumatoid arthritis, multiple sclerosis and Crohn's disease. [Nature Medicine, 1, 211-214 (1995) and ibid., 244-248].
Strong interest in drugs capable of selective inhibition of PDE 4 is due to several factors. Tissue distribution of PDE4 suggests that pathologies related to the central nervous and immune systems could be treated with selective PDE-4 inhibitors. In addition, the increase in intracellular cAMP concentration, the obvious biochemical consequence of PDE-4 inhibition, has been well characterized in immuno-competent cells where it acts as a deactivating signal.
Recently the PDE4 family has grown to include four subtypes—PDE4A to PDE4D, each encoded by a distinct gene (British Journal of Pharmacology; 1999; v. 128; p. 1393-1398).
It has been demonstrated that increasing cAMP levels within these cells results in suppression of cell activation, which in turn inhibits the production and release of pro-inflammatory cytokines such as TNF-α. Since eosinophils are believed to be a critical pro-inflammatory target for asthma, identification of the expression of the PDE-4 gene family in eosinophils led to the PDE-4 as a potential therapeutic target for asthma.
The usefulness of several PDE-4 inhibitors, unfortunately, is limited due to their undesirable side effect profile which include nausea and emesis (due to action on PDE-4 in the central nervous system) and gastric acid secretion due to action on PDE-4 in parietal cells in the gut. Barnette, M. S., Grous, M., Cieslinsky, L. B., Burman, M., Christensen, S. B., Trophy, T J., J. Pharmacol. Exp. Ther., 273, 1396-1402 (1995). One of the earliest PDE-4 inhibitors, Rolipram™, was withdrawn from clinical development because of its severe unacceptable side effect profile. Zeller E. et. al., Pharmacopsychiatr., 17, 188-190 (1984) which is herein incorporated by reference in their entirety. The cause of severe side effects of several PDE-4 inhibitor molecules in human clinical trials has recently become apparent.
There exist two binding sites on mammalian PDE-4 at which inhibitor molecules may bind. Also PDE-4 exists in two distinct forms which represent different conformations. They are designated as High affinity Rolipram binding site PDE-4H and Low affinity Rolipram binding site PDE-4L [Jacobitz, S., McLaughlin, M. M., Livi, G. P., Burman, M., Trophy, T. J., Mol. Pharmaco., 50, 891-899 (1996)]. It was shown that certain side effects (vomiting and gastric acid secretion) are associated with inhibition of PDE-4H whereas some beneficial actions are associated with PDE-4L inhibition. It was also found that human recombinant PDE-4 exists in 4 isoforms A, B, C and D [Muller, T., Engels, P., Fozard, J. R., Trends Pharmacol. Sci., 17, 294-298 (1996)]. Compounds having more PDE-4D isoenzyme selectivity over the A, B or C isoenzymes have been found to have fewer side effects than Rolipram [Hughes. B et. al., Br. J. Pharmacol. 1996, 118, 1183-1191]. Therefore, selective inhibitors of PDE-4 isozymes have therapeutic efficacy in the treatment of inflammatory diseases, such as asthma and other respiratory diseases, without the undesirable side effects of prior non-selective PDE-4 inhibitors.
Although several research groups all over the world are working to find highly selective PDE-4 isozyme inhibitors, so far success has been limited. Various compounds have shown PDE-4 inhibition.

SmithKline Beecham's “Ariflo” of the formula A, Byk Gulden's Roflumilast of the formula D and Bayer's Bay-19-8004 of the formula E have reached advanced stage of human clinical trials. Other compounds which have shown potent PDE-4 inhibitory activity include Celltech's CDP-840 of the formula B, Schering Plough's D-4418 of the formula C, Pfizer's 5CP-220,629 of the formula F, Parke Davis's PD-168787 of the formula G and Wyeth's Filaminast of the formula H. However, it is believed that due to efficacy and side effects problems, Ariflo, CDP-840 and Bay-19-8004 were discontinued from clinical trials as a treatment for asthma. Other compounds of the formulae C and F are presently undergoing phase-1 clinical trials.
International Publication Nos. WO 2004/037805 and WO 2004/089940 disclose tricyclic compounds useful for the treatment of inflammatory and allergic disorders.